Mitochondrial DNA (mtDNA) molecules from different rats (Rattus norvegicus) of the SASCO colony are of two kinds (SASCO-1 and SASCO-2), and mtDNA molecules of Utah wild R norvegicus (Wild-UT) are of a third kind in regard to their sensitivity to various restriction enzymes, indicating that nucleotide differences exist among these mtDNAs (at least 1.8% between SASCO-1 and SASCO-2). HindIII cleaves all three mtDNAs at similar positions. Four single nucleotide differences were found among pBR322-cloned HindIII F fragments (169 nucleotide pairs) of SASCO-1, SASCO-2 and Wild-UT mtDNAs. Other cloned HindIII fragments and subfragments are now being subcloned into M13 viral DNA and used to compare nucleotide sequences from regions of the three mtDNAs which code for rRNAs, tRNAs, and various presumptive polypeptides. These data should allow us to evaluate effects on function of the differences in nucleotide sequences among the three mtDNAs. Using mtDNA from rat liver, and from the fusion products of tissue culture cells containing SASCO-1 and SASCO-2 mtDNAs, we will attempt to identify and quantitate recombinant mtDNA molecules. MtDNA from progeny of crosses between SASCO-1 and SASCO-2 mtDNA-containing rats will be examined to evaluate the contribution of the male to the mtDNA of the organism.